An inhibitory function for JNK in the regulation of IGF-I signaling in breast cancer

Insulin-like growth factor-I receptor (IGF-IR) is frequently overexpressed in a variety of cancer types. Since many breast tumors and cancer cell lines overexpress IGF-IR, we tested IGF-I effects on chemotherapy-treated breast cancer cells. IGF-I protects from chemotherapy-induced apoptosis, suggesting that overlapping signaling pathways modulate IGF-I and chemotherapy treatment outcomes. Taxol and other chemotherapy drugs induce c-Jun N-terminal kinase (JNK), a kinase that conveys cellular stress and death signals. Notably, in this paper we show that IGF-I alone induces a potent JNK response and this activity is reversed by inhibition of phosphatidylinositol 3-kinase (PI 3-kinase) with LY294002 in MCF-7 but not T47D cells. Cotreatment of cells with chemotherapy and IGF-I leads to additive JNK responses. Using cells overexpressing Akt, we confirm that IGF-I-mediated survival is Akt dependent. In contrast, overexpression of JNK significantly enhances Taxol-induced apoptosis and inhibits IGF-I survival effects. Further, JNK attenuates anchorage-independent growth of MCF-7 cells. The inhibitory effect of JNK appears to be mediated by serine phosphorylation of IRS-1 (insulin receptor substrate) since both Taxol and IGF-I treatment enhanced Ser312 IRS-1 phosphorylation, while LY294002 blocked IGF-I-mediated phosphorylation. Taken together, these data provide a mechanism whereby stress or growth factors activate JNK to reduce proliferation and/or survival in breast cancer cells.